This invention relates to a movable wall for a brake booster wherein a diaphragm that divides a cavity of housing into a first chamber and a second chamber has an axial bead that axially urges a backing plate against a shoulder of a flange on hub member while a ledge on a front face of the axial bead is constrained between the backing plate and a groove in the hub member and a convolute that extends from a rear face thereof allows the diaphragm to move with the hub during a manual brake application without the introduction of a radial force on the axial bead that may effect a seal there between.
The movable wall of a vacuum brake booster usually consist of a backing plate and a diaphragm with the diaphragm having an external peripheral bead held between flanges on a front shell and a rear shell and an internal bead retained in a groove in a hub of the movable wall to separate a front chamber from a rear chamber, such as disclosed in U.S. Pat. No. 3,083,698. In an effort to eliminated the nut type connection is was disclosed in U.S. Pat. No. 3,754,450 that an internal bead having a smaller diameter than a diameter of the groove in the hub would allow the internal bead to resiliently snap into the groove to define a radial seal between the internal bead and the bottom of the groove. Further efforts to reduce the bulk or axial dimension of the movable wall, the radial flange of the hub was reduced and a resultant output force axially transmitted into the flange through the backing plate with the internal bead holding the backing plate against the hub in a manner as disclosed in U.S. Pat. Nos. 3,897,718 and 3,958,497. During a brake application, a first pressure is present the front chamber and a second pressure is present in the rear chamber and as a result a pressure differential is created across the movable wall that creates a force that is communicated through the backing plate into the hub for providing an actuation force to develop a braking force in a master cylinder to effect a brake application. This type structural arrangement functions in an adequate manner as long as a pressure differential is created across the movable wall. Unfortunately in an absence of an available first pressure (vacuum) for the front chamber, in order to effect a brake application, a manual input force must move the hub to provide the master cylinder with an actuation force to effect a brake application. In moving the hub by the manual input force, the diaphragm is stretched and places a radial force on the internal bead such that under certain condition it may actually separate from the groove and as a result it has been suggested that a keeper be attached to the hub to retain the bead in the groove. Unfortunately, some of the input force applied to effect the manual brake application is reduced by an amount equal to the force required to stretch the diaphragm.